The present invention relates generally to joining zinc-coated metal parts in a low porosity weld. More specifically, this invention pertains to a method of forming a weld between overlapping galvanized steel parts using a first laser to cut a slot through the top part to evaporate zinc at the weld location and a second laser to form the weld in the zinc depleted region.
Galvanized steel parts are used in the automotive industry for making auto bodies and other auto components because of their relatively high resistance to corrosion. In these applications, it is often necessary to form a linear weld junction between facing surfaces of two overlapping galvanized steel parts. A high energy laser beam is directed against a surface of one of the parts and along the weld path to momentarily melt a weld slot in the steel through to the interface of the parts. As the beam moves further along the path, previously melted metal loses heat to the adjoining metal and re-solidifies to complete the weld.
The difficulty with using laser beam welding for joining galvanized steel parts, however, is that the relatively low melting point of the zinc coating at the welding interface of the overlapping sheets vaporizes when the steel melt is formed. Some of the zinc vapor is trapped in the re-solidifying iron and the resulting weld nugget is porous and weakened. When there is no escape path for the zinc vapor, the final weld is often porous and unreliable.
Thus, it is an object of the present invention to provide a method of joining zinc-coated steel parts by laser beam welding to produce strong and durable welds with limited weld porosity.
The present invention provides a method of joining a galvanized steel part to another metal part using laser welding. Often both parts will be galvanized with a coating of zinc and both parts will be sheet metal parts. The parts are held together in an overlapping configuration and at least one layer of zinc coating is present at the welding interface.
The method of the present invention produces a weld between the overlapping parts by directing two laser beams in tandem along a linear welding path. The joining is accomplished by forming one or more welding seams between the overlapping parts. A first laser beam is directed and advanced along the exposed surface of the steel part at an energy level sufficient to pierce or cut a slot therein. The slot is cut to a depth extending through the upper part to a welding interface and, when desired, the welding slot may be cut into and through the bottom part as well. The slot is cut at a width sufficient to permit the zinc vapor generated by the high energy laser beam to escape from the slotted interface of the parts. In some welding situations it may be preferred to assist removal of the zinc vapor from the slot region by the flow of a suitable gas such as nitrogen.
Immediately after the slot has been cut into the steel part, a second laser beam is directed along the slot following the first laser beam. This second laser beam is of sufficient energy and width to melt portions of the metal adjacent the slot. This molten metal flows into the slot and solidifies to constitute the filler metal or weld nugget joining the parts. Since the zinc metal at the weld interface was expelled from the region by action of the first laser the weld nugget is free of pores and constitutes a strong weld.
As an alternative or supplement to using metal next to the slot as weld filler metal, a wire of filler metal (e.g., a steel wire) may be inserted between the second laser beam and the slot to provide a steady melt and flow of welding metal into the slot. Again, after the second laser beam passes over the weld site, the molten material that enters the slot is quenched and solidified by the adjacent part material to form a non-porous weld nugget.
In general, it is preferred that the parts to be joined are supported so that the linear direction of the slot is not vertical. Rather, it is preferred that the tandem laser beams be directed generally downwardly for best management of the removal of the zinc and the refilling of the weld slot with molten metal.
These and other objects and advantages of this invention will become apparent from a detailed description of the preferred embodiment that follows.